1. Technical Field
The present invention relates to a magnetic data processing apparatus, a magnetic data processing method and a machine readable medium containing a magnetic data processing program. More particularly, the present invention relates to a technology for updating an offset value of magnetic data outputted from a magnetic sensor.
2. Related Art
Heretofore, a method for updating an offset value of magnetic data is disclosed for example in Patent Document 1, Japanese Laid-Open Patent Application publication Patent No. 2006-53081. A track of magnetic data outputted by the magnetic sensor indicates a specific feature that varies depending on a carrier which carries an apparatus containing the magnetic sensor. Therefore, an optimum method of generating offset update data for updating the offset value varies in accordance with a manner by which the apparatus mounting thereon the magnetic sensor is carried (e.g., see Patent Document 1). One of the examples will be provided below. Namely, in a case where an apparatus on which a three-dimensional magnetic sensor is mounted is manually held by a person, a posture of the apparatus changes fast as compared with a case where the apparatus is held in a car. The posture of the magnetic sensor changes freely in a three-dimensional space when the magnetic sensor is carried by a person. Therefore, a large change in the magnetic data outputted by the three-dimensional magnetic sensor disposed on the manually held apparatus likely occurs even in a relatively short period. On the other hand, when the three-dimensional magnetic sensor is mounted on the car, the car substantially moves along a horizontal plane, and the posture of the car slowly changes during turning as compared with the posture change of the apparatus containing the magnetic sensor manually held by the person. Therefore, a large change in the magnetic data outputted by the three-dimensional magnetic sensor mounted on the apparatus held by the car does not likely occur in a relatively short period. When a distribution of the magnetic data for use in generating the offset update data is wide, the offset value is usually accurately updated. Therefore, the method of generating the offset value needs to be changed depending on types of carriers which carry the apparatus containing the three-dimensional magnetic sensor.
In the Patent Document 1, there is disclosed a method of monitoring a state of physical connection between an azimuth measuring apparatus and another vehicle-mounted apparatus. It is judged whether the azimuth measuring apparatus incorporating therein a magnetic sensor is being carried by a pedestrian of a vehicle or attached to a vehicle, or being carried by a person, to switch a computation mode of an offset value based on a result of the judgment.
However, the method disclosed in Patent Document 1 has a problem that hardware for monitoring the state of connection between the azimuth measuring apparatus and the vehicle-mounted apparatus is necessary. Moreover, even if a user is allowed to select the computation mode as disclosed in Patent Document 1, there is a problem that the usability of the apparatus is deteriorated.